or Cr₂K₂O₇ and its molecular weight is 294, while that of sodium dichromate, which is similar in constitution, but crystallises with 2Aq, is 298. The molecular weight of CrO₃ is 100. Chromic acid, and acidified potassium dichromate are powerful oxidising agents, and are used as such in many processes, and especially in the manufacture of alizarine. If sulphuric acid be used in molecular proportions, the product of the reaction is chrome-alum: 4H₂SO₄ + Cr₂K₂O₇ = 3O + 4OH₂ + K₂Cr₂(SO₄)₄. This, like ordinary alum, crystallises with 24Aq, and hence has a molecular weight of 998. It forms dark purple, almost black crystals, which are a fine garnet-red by transmitted light. In cold water it dissolves to a violet solution, which becomes green on boiling, but very slowly resumes the violet condition when cold. This change, which is not uncommon in chrome solutions, is probably due to a partial decomposition into free acid and a basic salt, the basic salts of chromium being generally green. It has been noticed that raw pelt swells much more in the green, than in the violet solution. Being derived from waste products, chrome-alum is often a cheap and valuable source of chromium for chrome tanning. For the analysis of chrome compounds see L.I.L.B., p. 141 _et seq._ Chrome oxide, and basic chrome salts, when strongly ignited, become insoluble even in concentrated acids, and their analysis is therefore attended with some difficulty. If, however, the ignited residue (for instance a leather-ash) be finely powdered, and intimately mixed with a fusion-mixture consisting of equal parts of pure calcined magnesia and pure dry sodium carbonate, and ignited (preferably over a Teclu burner), in a platinum crucible, in which it is occasionally stirred with a platinum wire, it will be quantitatively converted into chromate, which may be dissolved in acid, and estimated with potassium iodide and thiosulphate in the usual way. If it is desired at the same time to estimate sulphuric acid, it is sometimes preferable to substitute lime or calcium carbonate for the magnesia, which is apt to be contaminated with sulphates. Chrome is not only of importance in tanning, but in dyeing; on account of its power of forming insoluble colour-lakes with many mordant colouring matters. For this purpose normal or basic chromic salts are sometimes used, sometimes chromic acid or dichromates, the latter acting not only by yielding chrome-oxide on reduction, but as oxidising agents to the colouring matters. Most of the colours produced with chrome mordants are of dark shades, that with logwood being deep violet or black. The mordanting power of chromium is important in the dyeing of chrome leather. Bichromate of potash is often used in dilute solution for darkening the shade of leather dyed with other materials, but is not to be recommended on account of its destructive action on the leather.